Conventional drop-on-demand inkjet printers are commonly categorized based on one of two mechanisms of drop formation. A thermal bubble inkjet printer uses a heating element actuator in an ink-filled chamber to vaporize ink and create a bubble which forces an ink drop out of a nozzle. A piezoelectric inkjet printer uses a piezoelectric material actuator on a wall of an ink-filled chamber to generate a pressure pulse which forces a drop of ink out of the nozzle. Inkjet printers can also be categorized as multi-pass or single-pass printers. In multi-pass, or scanning-carriage inkjet printing systems, printheads are mounted on a carriage that moves back and forth across stationary print media as the printheads deposit or eject ink droplets to form text and images. The print media advances when the printheads complete a “print swath”, which is typically an inch or less in height. In single-pass, or page wide array inkjet printing systems, multiple printhead dies are configured in a printhead module called a “page wide array”. Thus, print swaths spanning an entire page width or a substantial portion of a page width are possible, which significantly increases the print speed of inkjet printers.
Monitoring the health of ink nozzles in the printheads is an important part of maintaining print quality in the thermal bubble, piezoelectric, scanning-carriage, and page wide array printers. Incorrect amounts of ink and inaccurate placement of ink on media by non-functioning nozzles can contribute to print quality defects. Causes for non-functioning nozzles include, for example, internal and external jetting head contamination, vapor bubbles within the jetting head, crusting of ink over the nozzles, a failure to activate the ink ejection element (e.g., resistive heating actuator, piezoelectric material actuator), etc.
Various methods of detecting failed nozzles have been developed. For example, sensors have been used in the past to detect whether a droplet has been ejected from a nozzle. In one method, a photo-diode and a light emitting diode (LED) sensor pair is used to detect the shadow of a droplet passing between the photo-diode and the LED. In another method, a piezo electric film is used as a droplet target to detect whether or not a droplet impacts the target. In another method, an electrostatic sensor detects a positive or negative charge from an ejected droplet. In yet another method, piezo-electric crystals are used to detect the acoustic signature generated as a droplet is ejected from the printhead.
Unfortunately, these and other methods of detecting failed nozzles have limitations. For example, such methods are unable to detect failed nozzles “on-the-fly” during normal fluid ejection activities, such as during printing. Because nozzle health can change during a print job or other fluid ejection routine, the inability to detect non-functioning nozzles on-the-fly (i.e., during a print job or other fluid ejection activity) can result in significant problems and added costs. This is especially true with page wide array printing systems used for large format or industrial printing applications. Page wide array printers often print extensive, long-run, roll-fed print jobs that can incur significant costs if the print jobs are interrupted to locate and correct non-functioning nozzles.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.